Without limiting the scope of the invention, its background is described in connection with transmitting downhole data to the surface during measurements while drilling (MWD), as an example. It should be noted that the principles of the present invention are applicable not only during drilling, but throughout the life of a wellbore including, but not limited to, during logging, testing, completing and producing a well. The principles of the invention are also applicable to bi-directional communication between surface equipment and downhole equipment.
A variety of communication and transmission techniques have been attempted to provide real time data from the vicinity of the bit to the surface during drilling. The utilization of MWD with real time data transmission provides substantial benefits during a drilling operation. For example, continuous monitoring of downhole parameters such as weight-on-bit, torque, directional surveys, and formation parameters in real time provides for a more efficient drilling operations. In fact, faster penetration rates, better trip planning, reduced equipment failures, fewer delays for directional surveys, and the elimination of a need to interrupt drilling for abnormal pressure detection is achievable using MWD techniques.
At present, there are four major categories of telemetry systems that have been used in an attempt to provide real time data from the vicinity of the drill bit to the surface, namely mud pressure pulses, insulated conductors, acoustics and electromagnetic waves.
In a mud pressure pulse system, the resistance of mud flow through a drill string is modulated by means of a valve and control mechanism mounted in a special drill collar near the bit. This type of system generates a pressure pulse that travels up the mud column at or near the velocity of sound in the mud. It has been found, however, that the rate of transmission of measurements is relatively slow due to pulse spreading, modulation rate limitations, and other disruptive limitations such as the requirement of mud flow. Common mud pulse systems are limited to data rates of approximately ten bits per second, with normal transmission rates of 2–4 bits per second.
Insulated conductors, or hard wire connection from the bit to the surface, is an alternative method for establishing downhole communications. This type of system is capable of a high data rate and two way communication is possible. It has been found, however, that this type of system requires a special drill pipe and special tool joint connectors which substantially increase the cost of a drilling operation. Also, these systems are prone to failure as a result of the abrasive conditions of the mud system and the wear caused by the rotation of the drill string.
Acoustic systems have provided a third alternative. Typically, an acoustic signal is generated near the bit and is transmitted through the drill pipe, mud column or the earth. It has been found, however, that the very low magnitude of the signal which can be generated downhole, along with the acoustic noise generated by the drilling system, makes signal detection difficult. Reflective and refractive interference resulting from changing diameters and thread makeup at the tool joints compounds the signal attenuation problem for drill pipe transmission.
The fourth technique used to telemeter downhole data to the surface uses the transmission of electromagnetic waves through the earth. It has been found, however, that in deep or noisy well applications, conventional electromagnetic systems are unable to generate a signal with sufficient magnitude to reach the surface.
In order to transmit higher data rates, repeaters have been proposed for both acoustic and electromagnetic systems. Installing in or connecting repeaters to the drill string have been described. See for example U.S. Pat. No. 6,075,461 to Harrison C. Smith, describing an electromagnetic repeater strapped to a drill string. These installations are highly susceptible to damage during drilling operations. Other devices, inserted in the drill string, typically limit the open diameter of the drill string that is required to run wireline tools and other safety equipment.
Therefore, there is a demonstrated need for a high data rate, non-intrusive system that is capable of telemetering real time information in a deep or noisy well between surface equipment and downhole equipment.